[ ♪ intro ] There are a lot of great rocks out there,
and we’ve even talked about some of them here on SciShow before. But in our opinion, some of the coolest rocks
are the ones that glow. Any rock will glow if you heat it up enough,
but there are a few types that are especially good at giving off light, and they do it in
all different ways. Here are five of our favorite rocks — and
how they get their glow. The most famous glowing rock might be fluorite, which is actually where the word fluorescence comes from. When this mineral is exposed to ultraviolet,
or UV light, it gives off an eerie blue, like your shoelaces under a blacklight. Essentially, it’s absorbing the UV light
and then re-emitting it at a different frequency. But fluorite and other rocks that fluoresce
tend to only absorb and emit specific colors of light. Shine the wrong color on them, and they just
look like an ordinary rock. And that’s actually a big hint about what
gives them their glow. Anything that’s sensitive to certain frequencies
of light has probably got something quantum mechanics-y going on. See, in every material, electrons are bound
to atoms in certain energy levels. And for an electron to step up or down from
one energy level to another, it has to gain or lose a specific amount of energy. Absorbing light is one way electrons gain
energy. But different colors of light have different
amounts of energy, so electrons can only absorb light with the exact right amount of energy
to take them up to a higher energy level. Once they get there, though, electrons don’t
typically stay there. They tend to lose energy and fall back to
a lower energy state. And as they fall back down, they often emit
photons of light. For fluorescent materials, you only need to
wait a nanosecond or so to see that happen. But the light they emit doesn't necessarily
have the same energy as the light they absorbed. Materials like rocks have lots of energy levels
in them, so an electron might lose a little energy to heat and drop down one level, and then
drop down the rest of the way by emitting a photon. So the photons they emit each have lower energies
than the ones they originally absorbed. That’s why you can shine UV light on fluorite,
but then see it glow in blue. And because UV light is invisible to humans,
it looks to us like fluorite glows in the “dark.” Overall, though, fluorescence is actually
pretty common: There are tons of fluorescent rocks out there, and they glow in all different
colors. And what’s amazing is they just look like
ordinary rocks until you see them under the right kind of light. Another mechanism that makes rocks glow is
called phosphorescence, and it was also named after a natural substance: phosphorus. Except, confusingly, while phosphorus does
glow, we now know it’s actually not from phosphorescence — it’s from chemical reactions
with the air. So that’s another story. But lots of other rocks do truly phosphoresce,
like calcite. Just like fluorite, calcite and other phosphorescent
rocks glow under UV light. But unlike fluorite, these rocks can truly
glow in the dark. Because even after you turn off the light,
they’ll keep glowing for a while. The reason for this is that phosphorescence
works almost exactly like fluorescence, but with one key difference. In phosphorescent rocks, electrons that have
jumped up to a higher energy level don’t fall down in a split-second the way they do in
rocks like fluorite. Their structure lets them hold on to that
energy for a lot longer. That’s because, before they drop down to
where they started, they usually have to go through what’s known as a forbidden transition. A forbidden transition is one that's just
very unlikely to happen. And it’s unlikely because it requires a
change in the electron’s spin—that weird, quantum mechanical property all particles
have that’s vaguely like ordinary rotation. And the thing is, transitions that involve
a change in spin are less likely than ones that don’t. Which means you have to wait longer to see
it happen. As a result, a phosphorescent rock that’s
been charged up will slowly release that pent-up energy as light over time, so you can see
it glow for minutes or even days since it was last exposed to light. Phosphorescence in nature is actually fairly
rare, because it’s hard to get a rock with forbidden transitions at just the right energies
needed to emit visible light in that way. But if you ever had glow-in-the-dark stars
on your ceiling at night, you’ve seen phosphorescence at work. Now, if you’ve tried shining a UV light
on a rock but you’re not having any luck, you’re not out of options. You can always try hitting it with something! That’s the essence of triboluminescence,
which is the type of glow you get when you hit a rock hard enough—or rub it, pull it
apart, or generally apply mechanical stress to it in some way. The term covers a lot of bases. Scientists actually don’t understand triboluminescence
perfectly, and it may work differently for different materials. But we have a decent idea of how it works
for rocks like quartz that give off light when you rub them together. As the rocks scrape against each other, some
charged atoms and electrons on the surface get knocked free. Then they collide with molecules in the air
and knock loose even more charged particles. As they recombine, those charged particles
create little electric shocks, like mini-lightning, that we see as flashes of light. But this doesn’t just happen with any rock. To get triboluminescence, scientists think
you need a rock whose crystal lattice structure is asymmetric in some way—in other words,
it’s not just made of nice, neat rows of atoms. That’s how some parts of the material end
up with excess electric charge, which can break free and produce that light when the
material is put under stress. Unlike fluorescence and phosphorescence, triboluminescence
tends to be a brief flash of light. And it’s a very common phenomenon—it’s
possible that around half of all inorganic compounds triboluminesce in some way, including
rocks like quartz and feldspar, as well as some everyday organic compounds like sugar. Today, scientists are still studying this
phenomenon and learning more about how it works. Which is pretty cool—because it’s not
often you can do science just by smashing rocks together! Now, as we mentioned at the start of this
episode, one way to make a rock glow is by heating it up. But heated rocks can actually glow for more
than one reason. Things like lava or hot coals glow because
of what’s called blackbody radiation: Heat makes their molecules vibrate, and that releases
energy in the form of light. But there’s another, slightly safer way
to get light from a rock by heating it, called thermoluminescence. This happens specifically when an object is
heated after absorbing energy in some other way. That energy usually comes from some kind of
radiation, like X-rays, but light, friction, and even pressure can sometimes do the trick,
too. Any of these processes can free electrons
from the crystal lattice that makes up a rock. But in thermoluminescent rocks, those freed
electrons quickly get re-captured in imperfections in the lattice. The thing is, these new bonds are weaker than
the original ones, meaning the electrons actually become easier to free again. So a little heat is enough to vibrate the
atoms making up the rock and set them loose. This time, the electrons fall back down into
their original positions. Once again, like in fluorescent rocks, those
electrons give off photons as they drop down. The amount of light these rocks give off depends
on how long they were exposed to the initial source of energy. The more electrons there are trapped in different
imperfections, the more light the rock will release when you heat it. And it takes only a little bit of heat to
release a lot of stored energy, so it’s actually pretty easy to get a bright glow
out of a thermoluminescent rock. For example, a rock called chlorophane will
visibly glow just from the heat of your hand. So if you pick up a rock and it starts glowing,
it probably doesn’t mean you’re the chosen one destined to save the world… Probably. Finally, if heating, lighting, and hitting
a rock still won’t make it glow, you can always try passing an electric current through
it. If it glows, that means your rock is electroluminescent. Just like heating or shining light on a rock,
passing an electric current through it is a pretty good way to charge up its atoms with
energy, as long as it conducts electricity in the right way. In electroluminescent rocks, the electric
energy is enough to free electrons from their atoms, leaving behind a bunch of positively-charged
atoms and negatively-charged free electrons. Since positive and negative attract, those
particles will quickly recombine and become neutral—and once again, you get that mini-lightning,
that quick release of light. The molecular structure needs to be just right,
so this doesn’t happen very often in naturally occurring rocks, but some colored diamonds
are an exception. See, in certain cases, diamonds aren’t
actually completely transparent. They can have flaws in their structure that
make them white or pink, and they can also have trace amounts of stray elements that
make them other colors, like yellow or blue. And those impurities are the key: Those little
interruptions in the crystal lattice are what emit light when the electric current passes
through them. What’s pretty wild about this is that the
electroluminescence of diamonds could potentially make them useful for quantum computing. That’s because quantum computing relies
on super precise control over individual particles. And a well-made artificial diamond with a
specific impurity can be sensitive enough to emit one photon at a time through electroluminescence,
which could help us build more robust quantum computers. So there are lots of glowing rocks out there,
and lots of weird ways that they glow. We actually didn’t get around to talking
about a few other neat examples, like how the radioactive element actinium glows as
its radiation reacts with the air around it. But y’know, there are plenty of safe ways
to get glowing rocks, so maybe leave actinium to the experts. Thanks for watching this episode of SciShow! And to find out even more about some of the
coolest rocks out there, you can check out our video on color-changing minerals right
after this. [ outro ♪]
